You can use the "Shade Tree Mechanic" method to get an approximate number. Raise the vehicle so the tires are off the ground and mark the tires with chalk or something like that. Crawl under it (after you've secured it with jack stands of course) and mark the driveshaft with chalk or something like that.
Spin the driveshaft by hand and count how many driveshaft turns it takes for the tires to make one rotation. Make sure that both tires make exactly one rotation.
This will give you a ballpark idea. For example, if it takes 4 and 1 tenth turns of the driveshaft per 1 tire rotation, then you could guess that you have 4.11/4.10 gears. As you might imagine, it's not the most accurate method.
If you pull the carrier out of the housing you can get a little more accurate. Mark the ring gear (the large one) and count the number of times you have to turn the pinion gear (the driveshaft yoke) to get the ring gear to make one rotation. Even this won't give you an exact number but it will get you closer.
The best way would be to find out what ratio came with your truck using the VIN. Unfortunately I don't know how you would do that. Maybe someone else can help you out there.